The present invention relates to improvements in a shift control apparatus of an automatic transmission, and more particularly to a shift control apparatus which enables a shift change from a neutral range to a forward shift speed range to be executed smoothly.
Generally, when a forward range of a manual valve is selected, an automatic transmission transfers a driver power from an input shaft to an output shaft by determining a power transmission passage of a gear transmission system through a selective engagement of friction elements such as a plurality clutches and brakes, and executes a shift change by changing the engaged condition of the friction elements. When a neutral range of the manual valve is selected, all of the friction elements are disengaged to put the automatic transmission in a neutral condition in which the drive power is not transferred from the input shaft to the output shafts. The automatic transmission set in the forward range executes an automatic shift change according to a vehicle traveling condition such as an engine load and a vehicle speed. Accordingly, when the shift range is changed from the neutral range to the forward range, a first speed gear of the forward range is selected to start the vehicle. However, such a shift change from the neutral range to the first speed gear of the forward range tends to generate vibrations or sink-down of the vehicle so-called a squat due to the radical raising up of the output shaft torque.
A Japanese Patent Provisional Publication No. 5-332436 discloses a shift control apparatus which is arranged to prevent the generation of the vibrations or squat of a vehicle. This conventional shift control apparatus is arranged to prevent a radical increase of the output shaft torque by once selecting a higher speed gear before selecting a first speed gear of the forward range. More particularly, working pressure is first supplied to a friction element for selecting a higher speed gear and is then supplied to a friction element for selecting a first speed gear while being discharged from the friction element for selecting the higher gear. A working pressure circuit for each friction element is respectively constituted by an accumulator, an orifice, a bent passage which are inherent by each circuit. Accordingly, it is not certain that the engagement and disengagement of the friction elements are executed by a predetermined procedure during the supply and discharge of the working pressure. This may generate striking noises between gears and shocks due to the engagement of the friction element for selecting the first speed gear preceding to the engagement of the friction element of the higher speed gear. In order to avoid such problems, this conventional shift control apparatus is arranged such that when a higher speed gear is selected preceding to the first speed gear, one of the friction elements for selecting the first speed gear or higher speed gear which one does not execute the transmission of the driving power is engaged, and the higher speed gear is then selected by the engagement of the friction element for selecting the first speed gear is engaged, and then the first speed gear is selected by disengaging the friction element for selecting the higher speed gear.
However, since it is necessary to quickly execute the supply of the working pressure to the higher speed gear friction element which was put in an empty condition in case of the change from the neutral range to the forward range, an oil pump consumes a lot of working oil and therefore the line pressure is temporally decreased. Although the temporal decrease of the line pressure does not cause a trouble in the supply process of the working pressure to the friction element of the higher speed gear, it causes shift shocks or squat in the supply process of the working pressure to the friction element for selecting the first speed gear if a fine pressure control is not executed. That is, since the friction element for the higher speed gear does not function independently to execute the power transmission, even if the supply of the working pressure is quickly executed, no trouble causes. Accordingly, even in a temporally lowered pressure condition such supply of the working pressure for selecting the higher speed gear is normally executed. In contrast, the engagement of the friction element of the first speed gear generates the power transmission. Therefore, it is necessary to finely execute the supply of the working pressure so as not to generate the engagement shocks and squat. That is the execution of the supply of the working pressure under the pressure temporally lowered condition generates the engagement shocks and the squat. Such troubles are significantly caused by the decrease of the discharge amount of an oil pump driven by the engine and by the decrease of the discharge amount of the pump. Further, since the circulation efficiency of the Working oil is degraded by the increase of the temperature of the working oil of the automatic transmission due to the increase of the leakage of the working oil from the oil pump, the troubles become significant according to the increase of the working oil temperature.
On the other hand, although the fine control under the pressure temporally lowered condition is ensured by means of the orifice and accumulator as mentioned in the prior art, such control increases the time period for selecting the first gear. This may generate a new trouble such as the degradation of a responsibility for starting the vehicle.